The present disclosure provides an epoxy resin wave-absorbing composite material and a preparation method thereof. The method includes: heating an epoxy resin to 50° C.˜70° C., and adding carbon black, to obtain a mixture of the epoxy resin and the carbon black; heating the mixture of the epoxy resin and the carbon black to 100° C.˜120° C., adding a curing agent, and stirring and dissolving them to obtain a mixture of the epoxy resin, the carbon black, and the curing agent; and adding surface-treated hollow glass microbeads into the mixture of the epoxy resin, the carbon black, and the curing agent, and curing them to obtain the epoxy resin wave-absorbing composite material.